How to block people on Telegram?
Via the contact info page: in a chat, tap on the profile image to enter the profile page of the user, then you will find different ways to block a user depending on which OS are you using:
iOS: Press “Block user” in the profile page.
Android: Press “…” on the top right corner, then “Block”.
WP: Press “…” on the bottom right corner, then “Block”.
Via Telegram Settings: “Privacy and Security” – “Blocked users” – “Add new…”.
How do I find out, if I have been blocked?
1) The profile image of the person who blocked you, is no longer visible. 2) Your messages always have only one check, never two. 3) “Last seen” or “online” status is no longer visible. Instead it says “last seen a long time ago”. 4) You can no longer invite the person to groups. 5) You will still see their messages in group chats and their typing status — and they will see yours.
Can I write blocked contacts?
On iOS and Android the input field will be locked and a string with “Unlock” will be shown. So you will have to unlock the user before you can write again to the him.

Windows Phone does not hide the text input field, so one can still write to a blocked contact.